According to the preamble of claim 1, the invention relates to a method for control of an automatic transmission.
In the electronic control device of modem automatic transmission, several shifting programs are usually stored. One shifting program usually contains a set of shifting characteristic lines. Each shifting characteristic line contains for a certain gear change, such as from third gear to fourth gear, the information as at what speed or rotational speed and at what load input or accelerator pedal position the gear change is triggered.
DE 39 22 051 02 discloses a method in which a gradient signal is determined from the signal of the accelerator pedal position and, depending on the value of said gradient, a performance optimized shifting program is adopted. This method is adequate to make a shifting program, with high shifting rotational speeds and corresponding traction excesses which can be desired, e.g. during an overtaking operation, spontaneously available to a driver.
The basic problem of an automatic transmission consists in that after triggering the gear shift a certain period of time elapses until the change of ratio. This period of time is needed, in electrohydraulically controlled transmissions, for filling a clutch to be engaged and/or for lowering the pressure level of a disengaging clutch. Particularly, when the driver quickly increases the load and expects a downshift, this time period is disturbing, since the transmission noticeably reacts only with a delay.
By a method such as disclosed, e.g. in DE 196 48 383 A1, the time period can be shortened within certain limits in order to increase the spontaneity of the transmission.
The problem to be solved by the invention is to achieve a further increase of the spontaneity of the transmission.
According to the invention this problem is solved by a method having the features of the characteristic part of the main claim. Advantageous developments of the method are given in the sub-claims.
With the aid of a gradient signal, the speed of change of the performance control device (accelerator pedal) is monitored. If the value of the gradient signal, during a quick depression of the accelerator pedal, exceeds a presettable threshold value, a downshift is triggered immediately before the shifting characteristic line has been exceeded. The transmission reacts prior to the time period which otherwise would elapse before exceeding the shifting characteristic line. In a driving situation in which, departing from a low load position of, e.g. 10% of the full load position, the accelerator pedal is quickly depressed and the downshift would be triggered only at a high load position, e.g. of 90% of the full load position, a shortening of the reaction time of the transmission of about 200 ms can be achieved.
The invention makes use of the knowledge that when the accelerator pedal is depressed at a specific speed, as a rule, it is actuated to a high load position which, in any case, would cause a downshift as a consequence.
In the less frequent case in which the characteristic line after the appearance of a high gradient value is still not exceeded, in an advantageous development of the invention, the downshift is again interrupted when the actual value of the gradient signal falls below another threshold value before the shifting characteristic line has been exceeded. Therefore, when the actuation speed decreases and falls below the other threshold value, it is concluded that the shifting characteristic line has probably not been exceeded and the downshift is interrupted. In an overlapping gear shift, where one clutch is engaged and one clutch disengaged, the interruption of the downshift is usually possible up to the moment in which slip appears on the disengaging clutch. The threshold value below which the downshift is again interrupted can be, e.g. equal to the threshold value above which the downshift is triggered, whereby a simplification of the adaptation results.
The adjustment of the threshold values has considerable influence on the behavior of the transmission. In order better provide for the behavior of different types of drivers, it is advantageous if the threshold values for the gradient signal depend on a driving activity parameter which usually is brought up for changing between different shifting programs.
When the threshold values for the gradient signal can be preset according to the actual gear, a further improvement of the transmission behavior can be achieved. For example, it is convenient to preset higher threshold values for the lower gears in order to prevent undesired downshifts which have a stronger effect in the lower gears on account of the greater ratio change.
As a consequence of the gradient threshold values having been exceeded, when the downshift is triggered only when the transmission temperature is within a presettable threshold range, the disadvantage of undesired high engine rotational speeds in cold or very hot transmissions is prevented. The limited value range of the transmission temperature is between 30xc2x0 C. and 150xc2x0 C., for example.
Under normal conditions, the output rotational speed of the transmission is proportional to the vehicle speed. When the downshift is triggered only within a presettable value range of the output rotational speed by the proposed function, undesired downshifts are prevented during low speeds usually used within small villages and at very high expressway speeds.
One other advantageous limitation of the state in which a downshift is triggered by the proposed function is given by a presettable value range of the engine torque. The triggering of a downshift only makes sense when operational traction is also present and therefore the engine torque is positive and exceeds a certain value, i.e. 30% of the full load.
Finally, inadmissibly high engine rotational speeds can be prevented by the fact that the downshift already is triggered before exceeding the shifting characteristic line only when the gear to which it should be downshifted is not lower than a gear which still can be introduced during the actual output rotational speed, without producing inadmissibly high engine rotational speeds.